Average January temperatures are −10 °C (14 °F) in the north and 3 °C (37 °F) in the south Yellow Sea area. The water temperature is close to freezing in the northern part in winter, so drift ice patches and continuous ice fields form and hinder navigation between November and March.
http://en.wikipedia.org/wiki/Yellow_Sea
Batteries are less efficient in the cold. All batteries deliver their power via a chemical reaction inside the battery that releases electrons. When the temperature drops, the chemical reactions happen more slowly and the battery cannot produce the same current that it can at room temperature. A change of ten degrees can sap 50 percent of a battery's output. In some situations, the chemical reactions will happen so slowly and give so little power that the battery will appear to be dead when, in fact, if it is warmed up, it will go right back to normal output.